The referenced U.S. Pat. No. 4,380,221 describes a combination fuel injection and distributor pump, for example suitable for automotive-type Diesel engines, in which a pump piston is reciprocated in conventional manner, for example by a cam, and the quantity of fuel is controlled by selectively opening a bypass or drain line, during the return pump stroke, and positioning a closing element with respect to the bypass duct in the piston so that a drain connection is opened at selected instances in dependence on the position of the closing element. The position of the closing element itself is controlled by a fuel quantity control element, for example an eccenter, engaging the slider, for example in form of a collar, to position the collar with respect to the bypass duct so that it will be exposed to permit drainage of pressurized fuel earlier or later in dependence on the respective axial position of the collar.
The fuel quantity positioning element, typically the collar surrounding the injection piston in form of a ring slider, is coupled for positioning by a rotary magnet, which controls the position of the eccenter, and forming the fuel quantity control element. The position of the slider with respect to the bypass duct must be accurately adjusted so that the quantity of fuel is properly associated with the movement of the injection piston. Since the pump and distributor combination provides fuel under substantial pressure, it is necessary to seal the positioning housing with respect to the fuel portion by seals which have the tendency, in due course of operation, to deteriorate and hence to leak. The seals and gaskets, thus, may require replacement from time to time. During replacement, it is necessary to remove the housing of the positioning structure from the housing of the pump structure itself, and then reassemble the housings together, after replacing sealing rings or gaskets. Upon re-assembly, it has been necessary to re-calibrate the positioning element arrangement with respect to the stroke of the piston, which is time-consuming and, effectively, can be carried out only on a test stand.